Planetary-period oscillations in Saturn’s magnetosphere
thesisposted on 10.01.2012, 15:40 by David Jeremy Andrews
This thesis is concerned with the study of near-planetary rotation period oscillations in Saturn’s magnetic field. Similar oscillations are present in a variety of other magnetospheric phenomena, most notably kilometre-wavelength radio emissions, and together they have a substantial influence throughout the Saturnian plasma environment. However, their origin remains poorly understood, despite more than a decade of study. Surprising new discoveries have been made regarding the seasonality of these oscillatory phenomena, and their intrinsic connection to the high-latitude upper atmosphere and ionosphere of the planet, though a complete theory of their origin remains to be developed. In this thesis, three detailed studies of this phenomenon are presented, each of which uses magnetic field data recently obtained by the Cassini spacecraft. The first consists of an extensive survey of near-equatorial field data during southern summer, from which the structure of the rotating oscillations is determined and the electrical currents flowing through the equatorial plane are calculated. The second study is prompted by the recent discovery of a north-south asymmetry in the period of related radio emissions, and consists of a survey of high-latitude magnetic field data in which evidence for a corresponding asymmetry in the magnetic field rotation period is found. Finally, the third study concerns the presence of long-termdrifts between the phase of the magnetic field oscillation and the Saturn kilometric radiation (SKR) modulation, which, through construction of a simple theoretical model, is shown to arise as a consequence of ‘rotational’ modulations in the SKR rather than the hitherto assumed ‘strobe-like’ modulations. These three studies substantially further our understanding of these phenomena, and in the case of the detection of a rotational modulation in the SKR overturn three decades of previous thought. Consequently, the results presented here elucidate characteristics of these phenomena that any theoretical understanding of their origin must serve to explain, and will therefore be of central importance in further development of this topic in the future. Before presenting these three studies, an introduction to the topics of space plasma physics and planetary magnetospheres is presented in Chapter 1. This is followed in Chapter 2 with a discussion of Saturn’s magnetosphere, and the topic of near-planetary period oscillations, and in Chapter 3 by descriptions of the instruments with which data used in this thesis have been obtained.